Photochemical [2 + 2] cycloadditions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص896-898
2025-07-23
377
Photochemical [2 + 2] cycloadditions
We shall now leave six-electron cyclodadditions such as the Diels–Alder and ene reactions and move on to some four-electron cycloadditions. Clearly, four is not a (4n + 2) number, but when we described the Woodward–Hoffman rules we used the term ‘thermally’. All suprafacial cycloadditions with 4n electrons are allowed if the reaction is not thermal (that is, driven by heat energy) but photochemical (that is, driven by light energy). Under photo chemical conditions, the rules switch such that all the cycloadditions that are not allowed thermally are allowed photochemically. This works because the problem of the incompatible symmetry in trying to add two alkenes together is avoided by converting one of them into the excited state photochemically. First, one electron is excited by the light energy from the π to the π* orbital.
Now, combining the excited state of one alkene with the ground state of another solves the symmetry problem. Mixing the two π orbitals leads to two molecular orbitals, and two electrons go down in energy while only one goes up. Mixing the two π* orbitals is as good—one electron goes down in energy and none goes up. The result is that three electrons go down in energy and only one goes up. Bonding can occur.
Alkenes can be dimerized photochemically in this way, but reaction between two different alkenes is more interesting. If one alkene is bonded to a conjugating group, it alone will absorb UV light and be excited while the other will remain in the ground state. It is difficult to draw a mechanism for these reactions as we have no simple way to represent the excited alkene. Some people draw it as a diradical (since each electron is in a different orbital); others prefer to write a concerted reaction on an excited alkene marked with an asterisk.
The reaction is stereospecific within each component but there is no endo rule—there is a conjugating group but no ‘back of the diene’. The least hindered transition state usually results. The dotted lines on the central diagram simply show the bonds being formed. The two old rings keep out of each other’s way during the reaction and the conformation of the prod-uct looks reasonably unhindered.
You may be wondering why the reaction works at all, given the strain in a four-membered ring: why doesn’t the product just go back to the two starting materials? This reverse reaction is governed by the Woodward–Hoffmann rules, just like the forward one, and to go back again the four-membered ring products would have to absorb light. But since they have now lost their π bonds they have no low-lying empty orbitals into which light can promote electrons. The reverse photochemical reaction is simply not possible because there is no mechanism for the compounds to absorb light.
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